buffer acid base help?

Question

What will be the effect of adding 0.5 mL of 0.1 M HCl to 100 mL of a phosphate buffer in which [H2PO4-] = [HPO42-] = 0.35 M?
The pH will increase slightly.
The pH will increase significantly.
The pH will decrease slightly.
The pH will decrease significantly.
Since it is a buffer solution, the pH will not be affected.

can youguys show me how to work wiff this problem, i apprecitate that. thanks

Answer 1

This is a buffer solution, and for all intent purposes, the pH will not be affected. An acid buffer solution involves a weak acid and its salt. This case is a bit tricky, because the "acid" involved is [H2PO4]- and the "salt" is [HPO4]-2.
BTW, put the ionic charges OUTSIDE the brackets. The math works the same. With the tribasic phosphoric acid, there are 3 Ka, each indicating where the ratio of "salt" to "acid" is 1.
In this case, we use Ka2 and
[H+][HPO4]-2/[H2PO4]- = Ka2.
In the solution, the conc of the "acid" and "salt" are equal. In 100 mL or 0.1 L of solution, there are 0.035 moles of each. I don't have the exact number, but Ka2 is about 1x10^-6, which is also H+, and the pH is 6.

The HCl addition provides 5x10-4 moles of H+. If the phosphate buffers WAS NOT there, the HCl would govern the pH of the solution, and the pH would drop to about 2.3. However, with the buffer, the "salt" reacts with the H+ and converts it to "acid", and the amounts of the "salt" and "acid" would become 0.0345 and 0.0355 moles respectively. The H+ would increase enough to re-establish the equilibrium, but if you check the numbers out, this is just a budge.

Note that buffers are not omnipotent. If you add enough acid (or base), you will wipe out one of the components of the buffer, and it will cease to be effective.